Semiconductor devices packaged in plastic or resin packages typically include portions of a lead frame. More specifically, each device includes a plurality of leads electrically coupled to a semiconductor die and a die support member on which the die is mounted. Throughout the industry, the die support member has many names. For the purposes of the present invention, the die support plate will be referred to as a flag. Many existing flags of semiconductor devices are in the form of a solid plate that is slightly larger in area that the semiconductor die and made of the same material as remaining portions of the lead frame, for instance, copper, a copper alloy, iron-nickel alloys, clad materials, and the like.
A problem with conventional plate-type flags is poor adhesion at an interface of the flag and the plastic package material. As a result of this weak interface, the plastic package material can easily separate from the flag leaving an air gap. Such an air gap becomes problematic upon mounting the device to a user substrate using conventional surface mounting techniques. Surface mounting techniques involve elevated temperatures that cause the air gap, which also may contain moisture, to expand. Stress built up in the package as a result of the air and moisture expansion is relieved by way of the formation of cracks in the plastic package body. Cracks in a package body are a path for contaminants to reach the semiconductor die, and therefore are a significant reliability problem.
Another problem associated with conventional plate-type flag is poor adhesion between the flag and conventional adhesive epoxies used to attach a semiconductor die to the flag. A die attach epoxy is generally dispensed onto the flag of a lead frame. Upon bonding a semiconductor die to the flag, the epoxy is dispersed, such that the epoxy forms a thin, continuous region beneath the entire die. While the epoxy typically bonds well to a surface of the die, the adhesion between the epoxy and flag is not as strong. Therefore, under certain stress conditions, there is a tendency for delamination between the die and the flag to occur. Such delamination raises reliability issues.
One solution, or at least improvement, to the package cracking phenomenon and to the problem of delamination between the flag and the die is the use of a window-frame flag. Rather than using a solid plate, a flag is shaped like a window-frame or ring having a central opening. Like the flag, the opening is square or rectangular to match the shape of a semiconductor die. The semiconductor die is mounted onto the frame flag, covering the opening. Ideally, the opening is as large as possible so that a maximum area of a surface of the die is exposed by the opening. Upon encapsulating the die and flag with a plastic package material, the portion of the die exposed through the opening in the flag is in contact with the plastic package material. The conventional lead frame and packaging materials used in semiconductor manufacturing are such that adhesion between a semiconductor die surface and a plastic package material is stronger than adhesion between a lead frame material and the plastic package material. The use of a window-frame flag reduces the possibility of forming an air gap in a package, and hence of forming cracks, since the area of an interface between the flag and package material is reduced in comparison to using a plate-type flag.
Although use of window-frame flags aids in resolving the problem of package cracking, the use of these flags leads to another problem associated with semiconductor packaging. In particular, a conventional window-frame flag creates an unacceptable potential for void formation during the encapsulation process. The voiding problem is explained below in reference to FIGS. 1 and 2.
Illustrated in FIG. 1 is a cross-sectional view of a mold tool 10 having an upper platen 12 and a lower platen 14. Mold tools, such as mold tool 10, are commonly used in the industry to mold a resin or plastic package body around a semiconductor die. When brought together, the upper and lower platens form a cavity 16 that defines what is to be a package body. A lead frame 18 is positioned between the upper and lower platens of mold tool 10 in a conventional manner. Lead frame 18 has a plurality of leads 20 and a window-frame flag 22. Within window-frame flag 22 is an opening 24. Positioned on flag 22 is a semiconductor die 26. Die 26 is typically attached to the flag using an adhesive material (not illustrated), such as a silver-filled epoxy. The die is electrically coupled to lead 20 by conventional wire bonds 28.
Using mold tool 10 during a transfer molding process to form a package body requires introduction of a plastic modling compound, such as a thermosetting epoxy resin, into cavity 16. The resin may be introduced into the cavity either from the top of the mold, the bottom of the mold, or from the side. Respectively, these molding operations are known as top-gating, bottom-gating, and side-gating. Regardless of where the resin is introduced into cavity 16, the resin must flow throughout the cavity in order to completely encapsulate semiconductor die 26, wire bonds 28, flag 22, and inner portions of leads 20. However, the frame shape of flag 22 impedes resin flow. FIG. 2 demonstrates, in an exploded view, resin flow near a flag region of lead frame 18 in mold tool 10 of FIG. 1. As a resin material 29 is introduced into cavity 16, the resin is diverted by die 26 and flag 22 so that a portion of the resin flows above the die and a portion flows below the die. Upon passing flag 22, the resin flowing below die 26 undergoes a boundary-layer separation, resulting in formation of a void or lack of resin in a region 32. The boundary-layer separation is a phenomenon associated with flow of a fluid perpendicular to a flat or sharp object. A void formed in a package body is similar to the delamination between a flag and a plastic package body in that both can lead to package cracking, and therefore both pose reliability concerns.
One method of avoiding void formation near interior edges of window-frame flags is to use a very slow curing, low-viscosity resin molding compound. During transfer molding operation, a mold tool is usually heated such that a thermosetting resin begins to cure and harden before encapsulation is complete. In using a slow-cure, low-viscosity mold compound, flow within a mold tool cavity is less restricted since it hardens gradually. However, a principal manufacturing goal of reducing cycle-time favors the use of fast-cure molding compounds. Therefore, another solution to the problem of void formation associated with using window-frame flags is needed.